1. Field of the Invention
The present invention relates generally to a lens fixing structure suitable for use when fixedly holding a plastic lens or the like and, more particularly, to a plastic lens fixing structure capable of ensuring a high accuracy against changes in ambient environment, especially, variations in terms of temperature and humidity in the case of requiring a compact configuration and a positional accuracy as in a focus detecting device of a camera, etc.
2. Related Background Art
A variety of principles about, e.g., a focus detecting method have hitherto been proposed. The fundamental principle thereof will be, however, explained briefly with reference to FIG. 8 with respect to the focus detecting device in such a form that images formed by a plurality of lenses are compared by arranging the plurality of lenses at a fixed interval.
That is, the numeral 10 in FIG. 8 designates an exit pupil of a photographing lens. The exit pupil 10 of this photographing lens is partitioned into a plurality of areas 91, 92. A field mask 20 is provided in the vicinity of a predetermined focal point of the photographing lens. A condenser lens 30 is disposed in rear of this field mask 20.
Further, a stop mask 40 provided posterior to this condenser lens 30 forms a pair of apertures 41, 42 corresponding to the two areas 91, 92 of the exit pupil 10 of the photographing lens. Provided in rear of the stop mask 40 is a plate-like member 50 serving as a re-imaging lens having a pair of re-imaging lens elements 51, 52 corresponding to the apertures 41, 42. Disposed posterior to this plate member 50 is a photoelectric converting element array 60 serving as a focus detecting unit including a pair of light receiving elements 61, 62 for re-forming light beams penetrating the lens elements 51, 52 as secondary images. Note that the symbol O represents an optical axis of the photographing lens.
Then, in the above-described configuration, the light beams passing through the areas 91, 92 of the exit pupil 10 of the photographing lens form a primary image in the vicinity of the field mask 20. Also, this field mask 20 regulates the light beams toward the condenser lens 30 and thus regulates a focus detection area.
Further, unnecessary light beams of those penetrating the condenser lens 30 are regulated by the apertures 41, 42 of the stop mask 40, which correspond to the areas 91, 92 of the exit pupil 10. Then, the light beams traveling through these apertures 41, 42 are re-imaged as secondary images on the light receiving elements 61, 62 of the photoelectric converting element array 60 through the lens elements 51, 52 of the plate member 50. A focusing state is detected from a relative positional deviation quantity between these secondary images.
By the way, the above-mentioned member having the plurality of lenses, e.g., the plate member 50 serving as the re-imaging lens is integrally formed of a typical plastic lens material such as an acrylic resin, a polystyrene resin and a polycarbonate resin. In this case, there is induced a problem in which a shape of the lens is deformed due to an influence of changes in the using environment such as a temperature and a humidity, resulting in an occurrence of error when detecting the focal point.
For example, the plastic materials given above generally exhibit a linear expansion coefficient larger by 3-4 digits (6.times.10.sup.-5 /.degree. C. in the case of, e.g., the acrylic resin) than that of a glass material with respect to the changes in temperature. The influence by the linear expansion coefficient is therefore unignorable.
Further, the plastic material stated above are rich in terms of a hygroscopicity and a permeability and therefore have such a nature that they are easy to expand with an increase in the humidity.
FIG. 9 shows one example of an influence exerted by environmental changes such as changes in temperature and humidity on the focus defection system based on the fundamental principle described above.
Explaining it briefly, to start with, the light beams reach positions 61A, 61B; 62A, 62B on a photoelectric converting element array 60 having light receiving elements 61, 62 in an initial state in FIG. 9. Herein, the symbols A, B designate the light beams passing through the same point on a primary imaging surface, and a 61A-62A interval is equal to a 61B-62B interval.
Further, herein, the plate member 50 is located at a lower edge in the FIG. 9 when attaching the plate member 50, i.e., the block lower edge of the plate member 50 serves as a fixed edge (FIX).
In such a state, however, a configurational variation derived from an expansion is produced due to the ambient environmental (temperature, humidity) changes. An exit angle of the light beams from a rear surface of the plate member 50 changes from the fixed edge toward a distal side. The positions in which the light beams reach the light receiving elements 61, 62 of the photoelectric converting element array 60 shift from the above-described ones to positions 61a, 61b; 62a, 62b in FIG. 9.
Then, such variation quantities of the light beam reaching positions increase as the positions get remoter from the fixed edge, with the result that a 61a-62a interval is obviously different from a 61b-62b interval.
Namely, when employing the above-mentioned plastic material for the lens, there can not be avoided changes in terms of a curvature of the lens and an optical axis interval due to variations in the environment in use such as variations in the temperature and the humidity. As a result, this induces an error when detecting the focal point.
Especially, with respect to the influence by the above environmental changes, an electric circuit for analyzing the data about an image detects a temperature and a humidity and is capable of compensating them in principle.
In response to demands for down-sizing of the structure and improving the productivity, however, when the plastic lens is fixed by use of the adhesive agent, and if the environmental changes such as the changes in the temperature and humidity are repeated, a reproducibility of the dimension of the lens interval worsens, and the compensations of the temperature and the humidity by the data processing circuit do not work effectively. The reason for this is that the shape of the plastic lens is restricted by the adhesive agent, and the adhesive agent alters complicatedly with a passage of time due to the repetition of the environmental changes.
As a mechanism for solving such a problem, e.g., Japanese Patent Laid-Open Application No. 1-92709 has hitherto already proposed a construction for obviating a level-difference in the light quantity of the light beams penetrating the lenses and a configurational difference in distribution of the light quantity by projecting a pattern member light quantity distribution on a photoelectric converting element array, extracting an electric signal representing the pattern member light quantity distribution by calculating an initial value with this processing and compensate-converting a relative level of the thus extracted electric signal per array.
However, according to the construction disclosed in Japanese Patent Laid-Open Application No. 1-92709, when compensating the humidity, there are needed an element for making a comparison with the initial state and a compensating element. These elements have to be incorporated into the apparatus, and, therefore, a structure of the apparatus becomes intricate. This brings about problems in which not only a size of the hole apparatus increases, but also the arithmetic processing for effecting the compensation becomes complicated. It is required that some measures be taken to obviate these problems.
Further, with respect to the deformation of the plastic lens due to the change in the temperature in the problems given above, Japanese Patent Laid-Open Application Nos. 61-65711 and 61-165712 pay attention to a method of fixing the plastic lens and disclose techniques of fixing the plastic lens by using an adhesive agent so that the plastic lens is expandable and stretchable in a free state.
Moreover, with respect to the deformation of the plastic lens due to the change in the humidity, Japanese Patent Laid-Open Application No. 5-107062 proposes a technique of employing low-hygroscopicity plastic as a lens material.
There exists, however, a possibility to cause the following inconveniences when using the technique disclosed in Japanese Patent Laid-Open Application No. 61-165711 or 61-165712 in order to relieve the influence by the deformation of the plastic lens due to the change in the temperature and, at the same time, when employing the technique disclosed in Japanese Patent Laid-Open Application No. 5-107062 in order to relieve the influence by the deformation attributed to the change in the humidity.
More specifically, the polyolefin resinous material is suitable for use with, e.g., optical parts among the low-hygroscopicity plastic materials disclosed in Japanese Patent Laid-Open Application No. 5-107062. The polyolefin resinous material has an extremely low adhesive strength with the adhesive agent. Hence, according to the construction shown in FIG. 10 but disclosed in Japanese Patent Laid-Open Application Nos. 61-165711 and 61-165712, bosses 17, 18 protruding from a base body 13 are connected to the plastic plate 50 defined as a plate member only at tips of the bosses 17, 18 through an adhesive agent 1. Accordingly, a sufficient connecting strength between the plastic plate 50 and the bosses 17, 18 can not be obtained, and there dan not be avoided such a possibility that the plastic plate 50 comes off.
Note that numerals 55, 56 represent boss fitting holes formed in the plastic plate 50, the numeral 40 designates a stop plate (stop mask), the numeral 65 denotes a focus detection unit based on the above-mentioned photoelectric converting element array 60, the numeral 15 designates a transparent cover thereof, and 16 represents a rear cover in FIG. 10.